I. Field of the Invention
This invention relates generally to electronic test apparatus for measuring the output signal characteristics of a pulse generator, and more specifically to the design of a circuit which permits voltage or current impulses of a variable duration and unknown amplitude to have their amplitude measured at a desired location along the time axis thereof.
II. Discussion of the Prior Art
In known prior art voltage and current amplitude test apparatus it is the usual practice to measure the peak current or voltage magnitude. Typically, where square wave pulses are involved, it is usual that the leading edge of the pulse will be of a greater amplitude than the trailing edge with a characteristic exponential droop exhibited between the leading edge and the trailing edge. In many instances, a person is more interested in knowing the average current or voltage amplitude than he is in knowing the peak amplitude. As such, more meaningful information is obtained if the measurement is made at a point along the time axis which is between the leading and trailing edge, e.g., at the midpoint of the pulse width.
As will be set out in greater detail hereinbelow, the present invention was specifically conceived for use in a hand-held, portable, battery powered test instrument useful in evaluating the performance of cardiac pacing devices and in determining pre-implant threshold data. The test instrument is adapted to be coupled to a cardiac pacer pulse generator so as to receive the output pulses from the pulse generator. In evaluating battery power consumption, it is much more meaningful to know the average current amplitude of the pacer pulse rather than the peak amplitude. The same comment holds true with respect to voltage measurements and resistance measurements. That is to say, by sampling the voltage and current amplitudes of pacer pulses at, for example, the midpoint of the impulse, the readings prove to be more representative of the average values than is true when leading edge measurements are employed. In that the resistance seen by the pulse generator is obtained by dividing the voltage by the current, a midpoint reading proves to be more meaningful. Then too, the resultant current at the midpoint is generally lower in each case than it is at its leading edge and is also more representative of the pulse generator's average current drain during pacing. As such, the user is more able to measure and predict battery life knowing the pacer's average operating characteristics as distinguished from its peak characteristics.